The present invention relates to a system and a method for improving conversion efficiency of an active lean NOx catalyst in a diesel or lean burn gasoline engine, and, more particularly, to improving its conversion efficiency via controlled use of a NOx reductant.
Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to reduce carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation. Vehicles equipped with diesel or lean burn gasoline engines offer the benefit of increased fuel economy. These vehicles may be equipped with lean exhaust aftertreatment devices such as xe2x80x9cActive Lean NOx Catalystsxe2x80x9d (ALNC). In order to achieve NOx reduction in the ALNC, a reductant, such as diesel fuel (HC) or urea, has to be added to the exhaust gas entering the device while the device is operating lean.
One approach to reducing NOx in the ALNC is described in U.S. Pat. No. 5,628,186, which uses urea to as a reductant. It teaches maintaining catalyst temperature within a predetermined range to achieve optimum NOx conversion efficiency. When the catalyst temperature exceeds a predetermined maximum temperature, it is cooled by an external cooler in order to bring it to the operating range where efficient NOx conversion is possible.
The inventor herein has recognized a disadvantage with this approach. In particular, the prior approach did not recognize that storing a reductant in the ALNC will greatly increase its NOx conversion rates efficiency, and further, improved storage of reductant in the ALNC occurs at temperatures lower than the temperatures at which peak NOx conversion efficiency occurs. Therefore, the prior approach of keeping the catalyst temperature in the optimum NOx conversion temperature range does not utilize the full conversion benefits of stored reductant.
An object of the present invention is to provide a system and a method for improving the conversion efficiency of an exhaust gas aftertreatment device by utilizing conversion benefits of a stored reductant.
In carrying out the above object and other objects, features and advantages of the present invention, a system and a method for improving conversion efficiency of an exhaust gas after-treatment device located downstream of an engine include the steps of: periodically creating a first operating condition while an exhaust gas entering the device is at an air-fuel ratio lean of stoichiometry; and injecting a reductant into the device during the first operating condition while the exhaust gas is at an air-fuel ratio lean of stoichiometry.
In another aspect of the present invention, the above object is further achieved, and disadvantage of prior approaches overcome, by a method for improving conversion efficiency of an exhaust gas after-treatment device coupled to an engine, the method consisting of: maintaining the device in a first operating range to store a reductant while an exhaust gas entering the device is at an air-fuel ratio lean of stoichiometry; and subsequently maintaining the device in a second operating range to release the stored reductant, thereby reducing an exhaust gas component while the exhaust gas is at an air-fuel ratio lean of stoichiometry.
An advantage of the above aspect of invention is that by keeping the exhaust gas aftertreatment device at temperatures below optimum NOx conversion efficiency in order to allow a predetermined amount of reductant storage, and then raising the temperature of the device to optimum NOx conversion temperature, NOx conversion efficiency of the device will be improved and it will more fully utilize the benefits of stored reductant.
The present invention provides a number of other advantages. In particular, An advantage of the present invention is that improved emission control can be achieved by varying the temperature of the ALNC and optimizing both reductant storage and NOx conversion.